586 SCIENCE PROGRESS 



complex and the confusion and wasteful expenditure of energy 

 which would ensue were its opponent parts to obstruct each 

 others' efforts would seem foreign to Nature's usual harmonious 

 economy. Inhibitory as well as excitatory control of the 

 separate muscles would a priori appear a necessity. In recent 

 years evidence has been forthcoming that such inhibitory 

 control does exist and is habitually exercised. Of this control 

 and its employment this article has briefly to speak. 



The contractions tonic and alterative of the skeletal muscles 

 are, as said above, always directly incited by nervous discharge 

 from the central nervous system. If that discharge ceases the 

 contraction of the skeletal muscle subsides. To curb the con- 

 traction of a skeletal muscle all that is necessary therefore is 

 to quell the discharge of the motor centre, which, lying in the 

 central nervous system, emits impulses to the muscle. Each 

 muscle has a motor centre of its own which presides directly 

 over that muscle and is the only channel by which motor im- 

 pulses can reach it. But upon the motor centre, on the other 

 hand, many nerve-paths converge, transmitting to it nervous 

 impulses from various other centres and regions. Of these 

 nerve-paths some possess the power of exciting and activating 

 the motor centre in question ; others of inhibiting it and throw- 

 ing it out of action. These latter curb the contraction of the 

 muscle by quelling the discharge of motor impulses from the 

 motor centre. The motor centre lies as it were an instrument 

 passive in the hands of these opposing forces of excitation 

 and inhibition exerted by the nerve-channels which reach it. 

 Sometimes the one influence and sometimes the other influence 

 is dominant; often the two are simultaneously in action, and 

 then their opposed influences partially cancel in proportion to 

 the relative intensities of their respective stimulations. 



Just therefore as the mechanism intrinsic in the heart and 

 driving its beat is controlled by a double set of nerves passing 

 from the central nervous system to it — one set quickening and 

 increasing the beats, the other slowing and lessening them — so 

 the motor centre of each skeletal muscle is the field of meeting of 

 two sets of opposed nerve-channels which influence it in opposed 

 directions. But in the latter case the motor centre, the field of 

 collision of the two opposed forces, lies within the central nervous 

 system ; the nerves which reach it, instead of being centrifugal 

 and passing out of the central nervous system, are centripetal and 



